fracker



Nov. 16, 1937.- H. esl-QCKE 2,099,298

` ELEQTRILTESTINPYARAQQS' I Filed sept. 21 19:55 ssnggissneet 2- am Y i INVENToR.

ffm/a3 duw/ew BY A. k.

f 1 ATTORNEYS Nov.' 16; 1937.`

H. E. FRACKER ELECTRICAL TESTING APPARATUS Filed sept. 21, 1955` 5 sheets-shears AIIIEIIIlI 5 n ta Y .v4 m wr -w f o IMM .T 4. s u..." m, k rv l Kl |.V

l um r h LMI All fo, e 4 e e 0/v f1 j 7 ,fw .,...rn o w a n V v INVNTOR ATTORNEYS Nov. 16, 1937. H, E. FRAcm-:R 2,099,298 I ELECTRICAL TESTING APPARATUS l.

Filed sept. y21, 1935 5 sneetssheet 4 INVEN'roR.

v ATToRNEY Patented Nov. 16, 1937 Henry E. Fracker,

National Technical 14 Claims.

This invention relates broadly to'electrical testing apparatus and has particular application to the measurement of potentials in extremely high resistance circuits. A specific field in which it is especially useful is in the determination of the acidity or pH of solutions.

Many features of this invention may also be employed to advantage in colorimeters, densitometers, photometers, electric titrating devices and other apparatus in which potentials of extremely high resistance circuits are to be measured or utilized.

It has been found convenient in pH determination to employ a voltaic cell having as electrolyte the unknown solution to be tested, in which are immersed a pair of especially chosen electrodes,Y

at least one of which makes electrical contact with the electrolyte through a thin glass wall. The potential of the cell is a direct function of the pH of the electrolyte so that by measuring the potential the pH of the solution constituting the electrolyte may be determined. It is a problem to accurately measure the potentials of such cells because of their extremely high internal resistance, resulting from the use of the glass-electrode.

In copending application for United States Letters Patent of Arnold O. Beckman and Henry E. Fracker, Serial No. 748,048, filed October 12, 1934, there is described a. testing'circuit for accurately measuring the potentials of glass-electrode cells by the null method. In that circuit the cell to be measured is connected in series opposition with a direct reading potentiometer and the input circuit of a special voltage indicating amplifier hav ing a meter in its output circuit. The amplifier is designed to have an extremely high input resistance and indicates with suiiicient .accuracy when the potential of the potentiometer is equal to that of the cell being measured.

Abroad object of the present invention is to make the described system even more accurate and reliable, more nearly fool-proof and mor rapid and convenient to operate.

More specific objects ofthe invention are:

(l) To provide an accurate potentiometer which is simple in construction and which supplies potentials over a continuous range including both positive and negative values;

(2)`To provide va potentiometer that may be easily and quickly Vstandardized without changing the setting of they indicating dial;

(3) To provide a potentiometer for measuring pH voltage equivalents that is adjustable t9 indi- Pasadena, Calif., assig'nor to Laboratories, Pasadena, Calif., a corporation of California Application september 21, 1935, serial No. 41,603

cate pI-I values directly at several different temperatures; Y

(4) To reduce the error resulting from leakage currents in high resistance voltage indicating circuits;

(5) To reduce swinging of the indicating needle of a meter in the output circuit of a D. C. ampliiierswhen the input circuit is momentarily disconnected from a source of potential being measured; .10

(6) lTo provide a satisfactory method of varying the gain in a D. C. vacuum tube amplier em- .ploying a screen grid tube;

(7) To reduce errors resulting from grid current when employing a vacuum tube amplifier l5 as an indicating device in voltage comparison circuits.

Various-other'minor objects and features of the invention will be apparent from the following detailed description which refers to the drawings. .20

In the drawings:

Fig. 1 is a circuit diagram of a complete 'acidimeter, or pH measuring unit, in accordance with the invention;

Fig. 2 is a detail circuit showing one form of 25 potentiometer for supplying a range of potentials, including both positive and negative values;

Fig. 3 shows a further modification of the simple circuit shown in Fig. 2;

Fig. 4 is a further development of the circuit 30 shown in Fig.l 3, the circuit being arranged for standardization against a standard cell;

Fig. 5 is adiagram illustrating the relation between the pH of a solution employed as electrolyte in a measuring' cell and the potential across the 35 electrodes of the cell at different temperatures;

Fig. 6 is a schematic diagram of the potentiometer circuit shown in Fig. 1;

Fig. '1 is a diagram illustrating how the scale.

- v. voltage characteristic 'of a potentiometer may 4o be altered both as to slope and position to provide temperature compensation;

Fig. S-is a diagram illustrating one form of temperature compensation circuit for use with a potentiometer adapted to directly indicate pH 45 values; i

' Figs. 9 and l() illustrate modifications of the temperature compensating potentiometer circuit shown in Fig. 8;

Fig. l1 is a schematic drawing illustrating the 50 leakage paths in shunt to the high lresistance circuit of an` acidimeter employing a glass-electrode cell in combination with a potentiometer and vacuum tube amplifier;

yFig, 12 is a graph iuustrating the'efrect p ro- 56' duced eiiect on the plate current of the second tube of the two stage D. C. amplier disclosed in Fig. 1, caused by the momentary opening of the grid circuit of the rst tube;

Fig. 14 is a schematic circuit employed toexthe eiect of grid current in the circuit of g. Fig. 15 is a diagram illustrating the relation between the pH of a solution employed in a glass electrode-calomel electrode cell and the potential across the electrodes ofthe cell over the pH range from +9.5 to +125; and Y Fig. 16 is a view of a part of a potentiometer scale calibrated in pH units and adapted to cover the pH range up to +125.

A very general description ofthe complete circuit shown in Fig. 1 will rst be given and this will be followed by detailed descriptions 'of the Avarious new features, with reference `to the re-V maining figures of the drawings.

General description Referring to Fig. 1, the system therein disclosed includes a test cell I `which may comprise a glass electrode 3 and a calomel electrode 5 immersed vin a solution to be tested, which is contained in a beaker 1. 'I'he glass electrode 3 is connected by a conductor 9 to the front contact II ofv a transfer switch I3, which switch also includes a transfer contact I5 and a back contact Il. The transfer contact I5 is connected by a lead I9 directly to the positive terminal of a B battery 35 and is `also connected through a biasing battery 31 to the grid 39 of an output tube 4I, the latter constituting atriode having, in addition to the grid v 38, a lamentary cathode "and an anode 45.

Anode 45 is connected through an indicating milliammeter 4f! to the positive terminal of the B battery'35. Cathodes 25 and'43 of the two tubes are connected in series with each other and' an A battery 49 through contacts 48 and 5D of a control 5I. To improve the operating characteristics of the input tube23 the lament 25 is operated at a relatively low temperature;i

anatoihaendtnis cathode is shunted by a resistance.- Y

'me circuit from the grid lzi is completed baci to the cathode of that tube either through the front contact II lof switch I3, the cell-I, the potentiometer. I5 (to described in detail later) and the back contact G3 and transfer contact 65 of a switch Il, or through the back contact I'lV 11, 15, 8'I and 1 9 has as its function to provides'.

variable bias onthe grid return'lead 12 to compensate for the variations in the strengths of the A battery 48, the B battery 35 and the bias battery 31. This circuit is fully described in copending application Serial N o. 748,048 and per se does not constitute a part of the present invention.

'ro operaie the circuit or Pig. 1, the switch si the tap 'I3 on resistor 'I5 and the variable resistor The transfer switch I3 is then actutiometer 55 in series between the grid 2| and the grid return lead 1I; Now if the potentiometer 55 is adjusted to provide a potential equal in magnitude but connected `vin opposition to the potential of the Vcell I, then the meter 4l will give the same reading as it`did with the transfer switch I3 in normal position. If the same reading is not obtained, then the potentiometer 55 is adjusteduntil the system is balanced. When a condition of balance has been reached, as indicated by' no deviation ofv the needle of the meter 41, when thev transfer switch I3 is actuated, then the potentiometer indcatesthe tential of the cell I'.

The portion of the circuit so far described -corresponds closely -to the circuit disclosed in the aforementioned copending application and serves as -a necessary foundation for the proper Vunderstanding of the new features which are the subject matter of the present invention. These new features will now be takenl u p separately.

Potentiometer circuits for providing a 4range of value of the po- 'potentials including both-positive and negative values vsists of a glass electrode and a calomel electrode immersed in a-solution to be tested, varies with the -pH of the solution. Ifit is desiredto cover a pH range ofy from 'a -ito -`+13, thenv the E. M. F. of the cell will vary from .5115 volt to +2476 volt at a temperature of 20 C.- 0n account of the reversal of polarity of the cell, it is necessary to know'the polarity of the potential as well as its magnitude.

v Y 40 The E. M. F. produced by the cell I, which con- The ordinary potentiometer consists of a slide resistor having a battery connected across its end terminals and having its output or E. M. F.

terminals connected respectively to .one end of the slide wire resister and to the slide. with auch an arrangement, potentials varying between zero and desired positive value or .aero and any desired negative value may be obtained,

butiiisimpossibietoobiainfrom suchacifsrice- 5 a range of potentials including both. positive and negative values simply by lmanipulation of the slidingcontact. Hence it has been necessary,

when employing. the vordinary type or poten'. tiometer, to 'provide a .reversing switch in. conjunction therewith for reversing the polarity vof A F. supplied to the leads and 12, g Y

the E. M. (Fig. 1).

Obviously, it would be much more convenient calibrated directly in pH units instead of volts to have iii-potentiometer capable of giving any' 93h is immaterial insofar as the location of the v zero point on resistor 89h is concerned, this ,beingk (such calibration being permissible `since the lpotential corresponding to any pH value is constant at a. given constant temperature). u A simple form of potentiometer circuit that 1s capable of supplying a continuous range of potentials, including both'positive and negative'v values, is disclosed in Fig. 2 in which the ,con-

ductors 12a and 85a correspond, respectively, to the conductors l2 and 85 in Fig. 1. The circuit comprises a calibrated resistor 89a having connected thereacross a multi-cell battery 9Ia. This circuit differs from the conventionall circuit in that the lead 12a, instead of being connected to one terminal of the resistance 89a,'is tapped into the battery Sia at a point intermediate the ends thereof. The conductor 12a is thereby maintained at a reference potential intermediate the potentials of the opposite ends of the resistor 89a.v The siide am may be adjusted to a 'point on resistance 89a equal in potential to that of the lead 12a, thereby yielding zero potential across conductors 12a and 85a. On the-other hand, if the slide 81a is moved to the left, the conductor 85a will become increasingly negative with respect to conductor 12a, and Vice versa if the slide 81a is moved to the right along the resistor 89a,

conductor 85a will become increasingly positive with respect to conductor 12a.

In practice, the circuitshown in Fig. 2 has the substantial advantage over the conventional potentiometer circuit that it is very easy to obtain suffix b.

zero potential by merely adjusting the slide 81a to the nulll point. On the vother hand, in a conventional potentiometer, in order to obtain: zero potential, the ing clear to one end 'of the slide resistor so that no resistance is left between the slider and the end of the resistor. Note that in the circuit of Fig. 2, the resistance of ing 89a left in the circuit outside the maximum and minimum positionsv oi theslider does-not have to be zero. v

The circuit of Fig. 2, although advantageous in certain respects, has defects for use in an acidimeter circuit of the type disclosed in Fig. 1. Thus the potentiometer employed in the circuit of Fig. 1 need only cover a voltage range extending from .5115 to +2476 or a total range of .7591 volt. Therefore, a single dry cell supplying 1.5 volts connected to the potentiometer satisfies the voltage range requirements. Obviously, the method of locating the zero point intermediate the ends ofthe resistor 89a which is disclosed in Fig. 2, is impossible where the battery Sla comprises only one cell.

Another way of obtaining vthe desired result, while employing a single cell, is disclosed in Fig. 3, in which elements corresponding to the elements of Fig. 2 bearthe same In this circuit the slide resistor 89h is shunted `by a pair of additional resistors 92h and 93h, respectively, connected in series and the lead 12b is connected to the junction point of resistors 92h and 93b. If resistors 92h and 93h are made equal in value, then zero potential will exist across conductors 12b and 85h when the slide 81h is at the centerof the slide resistor 89h, and potentials of negative or positive value may be obtained by moving the slider to the left or to the right, respectively, of the center point. Obviously, by making resistors 92bvand 93h unequal, the zero point on resistor 89D may be shifted to the right or the left to obtain a greater range of negative potential than positive potential or vice ,tively stable potentials.. However,

slider must be capable of movthe potentiometer windy numerals but with the dependent only upon the relative magnitudes.

However, the resistances of resistors 92h and 93h are preferably kept high/in order to reduce battery drain and for another reason to be described later.

In the manufacture type `disclosed in Fig. 3, the zero point of rthe slider Blb may be readily located by making both the resistor 89band the slider 81h movable 4and attaching the calibrated scale to the resistor 89h.

vThe resistor may then be moved into the position Potentiometer standardization circuit As previously stated, it is desirable to employ as the battery Sib in Fig. 3 an ordinarydry cell since such cells are easily-obtained, are convenient for use in portable instruments, and, under conditions of low current drain, maintain relaforwork of any accuracy, it is necessary to standardize a potentiometer employing a dry cell against a of a potentiometer of theknown source of E. M. F. A standard cell of the Weston unsaturated. type oiers a convenient source of fixed known potential and may be used to provide a constant reference potential. Such a cell has no temperature error, so4 far as the magnitude of accuracy required in the acidirneter circuit of Fig. 1 is concerned, and if made properly and not subjected to excessive current drains, it should maintain its E. M. F. constant over a period of many years. The E. M. F. of such a standard cell is about 1.0188 volts which, it will be observed, is greater than .7591 volt required in my acidimeter, but is less than the potential of a standard dry cell (1.5 volts).

Heretoiore it has been the general practice in standardizing a potentiometer circuit to set the potentiometer scale to the known value of the standard cell and connect them in series with each other through a galvanometer and a key, the polarity of the standard cell being reversed with respect to that supplied by the potentiometer so that the resultant potential applied to the galvanometer will be zero lif the potentiometer is properly adjusted. If the potentiometer was not properly adjusted, as indicated by deflection of the galvanometer needle, then additional adjustable resistances in series with the potentiometer slide resistor and the potentiometer battery were adjusted to vary slide resistor until the potential drop between one end of the resistor and theA slide exactly equaled the potential of the standard cell.

This old methodhas two serious disadvantages: (1) it necessitates adjustment'of the `potentiometer seme to the potential of the standard een, thereby entailing an additionalcperation, which in turn necessitates still another additional operation, namely, resetting the potentiometer back to the test point following standardization of the potentiometer; (2) if the potentiometer is badly oif balance, a considerable current can flow through the standard cell while the circuit is closed and, since the current through .such a cell the potential range of the current through the of the usual size should not exceed 100 X10* am- Y corresponding to those inthe other iigures bear the same reference numerals but with the sufilx c. In this circuit an additional resistor 95e and a resistor 91e are connected between one end of the potentiometer slide resistor 89e and the potentiometer battery Sie, and a standard cell 99e is connected between the lower end of resistor 95e and front contact i02c of a switch 61e. Cooperat-V ing contact 59o of switch 61o isfconnected through lead 56e and contacts llc and lic of switch llc to the grid lead I9c of the amplifier. The grid return lead 12e of' the. amplifier is connected to Cil instrument are thrown .entirely out of adjustment contact 65o of switch 61e and the cooperating, front contact I08c is connected through lead lMc to the left end of the potentiometer resistor 89o. Actuation of switch 61e therefore connects the standard cell 99C, the resistor 95e and resistor 89c in series with the amplifier input leads i9c and 12e. f

The resistor o has a resistance such that the potential drop thereacross, added to the potential drop across the resistor 89c, just equals the potential of the standard cel1'(1.0188 volts) when l the current through the potentiometer resistor is adjusted tothe proper value. Therefore, if the current through resistor 89c1 is correct, no

-potential is impressed upon the amplifier input leads when switch 61o is actuated, and the output meter 41o so indicates. On the other hand, if the meter 41o indicates'an unbalanced condition, the current through resistor 89e is adjusted until a balance is obtained, by varying the resistor S'Ic.

The value of resistor 9 5c may be determined. foreach acidimete'rby substituting. a source of known potential for the cell I, settingthe potentiometer to its proper position corresponding to the known E.` M. F., and then adjusting the value of resistor 95e until the E. M. F. developed by the potentiometer is within a few tenthsof a millivolt of its proper value, when resistor 91chas been adjusted to a balance against the standard cell. The value of resistor c is determined when the acidimeter is assembled and is not changed thereafter.

It will bel observed that since the standard cell Y is balanced Vagainst the total potential d rop across resistors 89e and 95e instead of across the potentiometer output leads llc and llc, it is not vnecessary to alter the'position oi' the potentiomf ,eter slide in order toY standardize the. potentiometer, which considerably facilitates taking` po- Y tentiometer scale readings.

Furthermore, it should be noted that in the;

circuit of Fig. 4, and also the circuit of. 1, i

the standard cell is conectedin the circuit of the amplifier which has en -extremely highinput resistance, so that no appreciable current is drawn from the' standard celLthus allowingtthe lattertobemadeverysmallinsizeoompared to fthe usual standardcell; also it is impossible to draw an appreciable current from this cell re gardless of how badly out of balance the potentiometer may be evenif all the controls on the -so that the standard cell, which is essentially delicate, cannot be'injurd by unskilled operation A p of the acidimeter, a considerable improvement Y over prior practice in which the standard-cell l was simply connected in series with a galvanometer across the output terminals. of the potentiometer.

Two precautions must be observed in the'construction of the circuit shown in Fig. 4: iirst, resistors 89o and 95e must be wound with wire having thev same temperature coefcient so that as high temperature coeiilcients are used. If resistors 92o and 93e are made about 100 times as large as resistor 89e. then assuming that resistors 92o and'93c are wound with resistance wire having the highest temperature coefiicient of allcommonly used wires, th'e error introduced into the pH, measurements over an ambient temperature change of from 5? C. to 40 C. is a maximum of about .0413 pH. 'I'his error or source o! error would not be eliminated by the standardizing. operation and is the additional reason` previously referred to for making resistors'SZc and 93o high. It will be recalled that the other reason for making these resistors high was to reduce battery drain.

Temperature compensation circuits .In general, the E. M. F. developed by a. test cell I"comprising a glass electrode and a. calomel electrode immersed in an acid solution varies with changes in temperature, the E. M. F. increasing with temperature. It happens, however, that the -E. M. F. of the cell when the solution pH is +0.1

is constant (-0.4475 volt)irrespective of temperature. If the temperature is maintained con-v stant -but the pH varied. then the E. M. F. varies istics of the cell at temperatures of 5, 20 and 35 C. are shown in the graph of Fig. 5 in which the represent pH and the prdinates E. M. F,

It will belobservedthat all three characteristics intersect at the point located at +0.1 on the pH scale and 0.4475 on the E. M. F.V scale. The diiferences in the slopes of the three characteristics are exaggerated in the Yfigure to prevent'conthe t of the slide. It follows that for any.

given tanperatine the potentiometer scale may be calmndm pn una; instead of vousbuc'tne tnescale divisions win be dif- Vferent foro the'r The same potentiometer could' beused Vfor solutions atdifferent by providingseveral diiierent Hechanicdiy this is not a desirable ar rangement, and in accordance with the Vprenait.inventilout provide circuits for compensiting for different solution temperaone men circuit is shown in Fig. awmcn, uke

that ot Pig. 4, comprises a potentiometer slide resister "d, a sliding contact 11d-.connected to one with the resistor 93d and a tapped resistor |03d is inserted between the standardizing resistor 91d and the resistor 95d. A pair of switches |0511 and |0611 (preferably mounted on a common con- 4 trol shaft |01 for simultaneous movement) function to short out successive sections of the resistors |03d and 94d to adapt the device for use with solutions of temperatures at 5, 15, 25 and `35" C., respectively, While still employing only l a single potentiometer scale calibrated .directly in pH units.

The circuit of Fig. 8 functions as follows: Rel ferring to Fig. 5, it will be observed that as the temperature is increasedthe change in E. M. F. for any given change in pH increases. forev at a temperature of 20, for instance, a potentiometer graduated in pH units would have to vary its E. M. F. by a greater amount for a given pH scale movement than it would at a temperature of 5. Now since the E. M. F. developed by the potentiometer is directly propor-Y tional to the current through the slide resistor thereof, the increased E. M. F. can be obtained by simply increasing the total current through the potentiometer. Such an .increased current is obtained by cutting out a portion of the tapped resistor |03d. It must be noted, however, that increasing the total current through the potentiometer does not shift the point of zero potenc tial on the slideresistor since that is determined solely by the resistance of the resistor- 92d relative to resistors 93d and 94; it merely rotates the potentiometer characteristic about the zero axis. If the potentiometer is to indicate directly -in pH units at the temperatures of 5, 20 and 35 C.,. then when adjusted for those respective temperatures its E. M. F.`v. scale deilection characteristic must coincide with the E. M. F. vs. scale characteristics shown in Fig. 5,.

Referring now to Fig. '7, the line (a) represents both the E. M. F. vs. pH characteristic of the cell to'be tested at 5 and the E. M. F. vs. deflection characteristic of the potentiometer circuit in Fig. 8 when set for-5. I

|06d is moved'to the 20 tap on resistor |03d, the lslope of the potentiometer characteristic' is changedbut this change in slope is effected by -rotation of the characteristic about its point of intersection with the E.. M. F. base line, as shown by the line (b) in Fig. 7. Obviously, line (b) does not/coincide with line (c)` representing the cell characteristic for 20, but it can be made to coincide therewith by shifting its zero point, and

' in Fig. 8 this shifting of the zero point isleffected -by varying the resistor 94d through movement of the switch .|05dto the 20-tap. The E. M. F. vs. deection characteristic of the potentiometer then coincides with the E. M. F. vs. pH characteristic of the cell at 20. C. (line (c) inL Fig. 7)

and the point on the slide resistor corresponding to the pH of a +0.1 is restored' to its original potential of '-0.4475 volt.

In actual operation the switches |05d and `-|06d are shifted simultaneously and then the circuit is rcstandardized by adjusting resistor 91d while battery 9|d and resistor 01d are connected across a standard cell comparison circuit.

Therev Now if the ,switch l Although the circuit of Fig-8 gives correct results, it has the disadvantage (from a manufacturing standpoint) of requiring the two tapped and calibrated resistors 94 and |03d, which are relatively expensive. The tapped resistor 94 may be eliminated by employing the circuit shown in Fig. 10, in which elements corresponding to those reference numerals with the suiiix "e attached. The circuit of Fig. differs from that of Fig.

V8 in that resistor 94d is omitted and resistor 93e slide resistor so as to maintain constant the.

E. M. F. of the point on the resistor corresponding to a pH of +0.1 despite variations in the current through the slide resistor caused by adjustment of the switch |0Bd, thel latter adjustment being necessary to change the slope of the potentiometer characteristic. y

In Fig. 10 thev slope of the potentiometer characteristic is changed as before by manipulating the switch |066 and therebyyarying the current' through the slide resistor 89e. However, in Fig.

10 it is not necessary to perform another operation to shift the point of zero E. M. F., because the point |09e is automatically maintained at a potential of 0.4475 volt v(corresponding to a DH of +0.1) regardless of changes in the position of switch |06eand the value of the current in resistor 89e, by virtue of the fact that the point |09e is connected directly to the end of resistor 92e vand the potential drop across resistors 92e and 93e is always the same after the standardizing operation since it is this potential across resistors 92e and 93e that is compared with the standard cell. The magnitudes of resistors 92e and 93e are made so large as compared to the resistance of the slide resistor 89e that the current iiow through the resistor 89e to the left of the point |09e is not enough greater than that in the remainder of the slide resistor to cause errors of suiiicient magnitude that they need be considered. i,

Thel circuit of Fig. 10 has the mechanicalY disadvantage thatthe slide resistor, which is preferably rotatably mounted on a drum and cooper- .ates with a ystationary slide contact 81e, has to .be tapped at the point |09e, thereby necessitat- `ing a special spacing of the pHscale at that a pH range extending below +0.1, that portion of the slide resistor to the left of point |09e (Fig. 10) may be eliminated and the necessity of a tap avoided. Such a modication is shown in Fig. 9 in which elements lcorresponding to elements in Fig. 10 bear the same reference numerals but with the suix f.

It is desirable, however, in mou instances to in the previously considered figures bear the same l Y ist Y amount with respect to distance.

scheut-nung for n' 1n- 1) :L-

extend the pH range below +0.1 and a preferred circuit for accomplishing this without tapping the slide resistor is shown in the complete circuit of Fig. 1.

The potentiometer circuit 55 in Fig. 1 differs from that of Fig. 10 in that the slide resistor 89, instead of itself being provided with a tap 'connection, is shuntedby a stationary tapped resistor 90, the tap H0 thereon being connected to the left end of resistor 92 and to the conductor |04 leading to the standardizing switch 61. The remaining portion of the potentiometer corresponds exactly to that of Fig. 10 and the standardizing circuit corresponds to that previously described with reference to Fig. 4.

The standardizing operation must always be performed before the instrument is used and, as previously indicated, consists of iirst setting the temperature correcting vswitch |06 to the proper positionand then adjusting the resistor 91 until no deection of the milliammeter occurs when the switch 61 isactuated, thereby indicating that the potential across resistors 92 and 93 is equal to that of the standard cell 99.

As a result of the use of the tapped resistor 90, a xed point intermediate the endsof slide resistor 89 corresponding to a pH scale reading of, +0.1 may be maintained at a constant potential of 0.4475 Volt with respect to the lead 1I,- regardless of adjustments of the temperature correction switch |06, and the voltagel above. and below this point always varies linearly by the same This has been proved both experimentally and mathematically.

They mathematical analysis of the potentiometer set of Fig. 1 will now be given with reference to Fig. 6, which is a schematic diagram of the potentiometer circuit in Fig. 1 but with diiierent reference symbols correspondingto the symbols used in the equations.

. v fri-n trl-r4 From Equation 10 the following facts may be v deduced:

(a) The equation is linear; I1 and Ris ,are

a constant. Therefore, by properly choosing the values of Rp, 11,'1'3 and r4, `Iany desired point on the'potentiomet'er drum can be made to have a constant potential with respect to the junction of r3 and r4 irrespective of the current in Rp. For use in the acidimeter described, the point of fixed potential is chosen at a potential of .4475-vo1t corresponding tofa pH of +0.1.

(c) E can be made equal tozero but not independent of I1 if (from Equation 1 0):

Conclusions 1.-From Equation 10) E is a nnear function of I1 and Rus or of RLs alone when .I1 is ilxed,

since all of the other factors of Equation .(10) are constants of the circuit. I

2.-Also, .from Equation (10) E is independent of I, for the one case-when the slider is set so that i and E for that condition is equal to;

E.M.F. nrg +r) fri-r4 n+1: a which is always lnegative in sign for the given circuit arrangement.

3.-From Equation (l0) observe that E is zero when 11R, =E.M.F. r11: 11(12 IVI-fz) tari-r4 f Irl-131+r3 A.-From Equation (11) note that 'the position of the zero point on the pH scale is a linear function -ofIi and Ris, the two independent variables. For constant I1, Rm becomes fixed and vice versa.

`Also, if I1 is increased (the case when thetemperatureincrases and is compensated for) note that Ris is decreased, `or that the zero point will be llower on the pI-Lscale which corresponds exactly to the result obtained in Fig. 8.

V B.-From Equation (i1) and Conclusion 2 above,

observe that the expression within the brackets, i'. e.,

maybe set equal tonne, sineethe bracket expression -is equivalent to the distance. betweenv -the slider position and the point where E=O,

(point O infFig. 6) and is vlikewise equal therefore to the difference in pH values betweenthese two` points, Thus rewriting Equation (11):

' EMR 'an' IRO trl-vn ri-Ir.: From this, it is seen that Rxo is inversely proportional to I1, orthat thepH lc'a'irresponding to +,fs)=C (a constant) I I zero l decreases linearly and in inverse proporv end turn resistances on theends of the potentiometer winding outside the scale ends do not enter at all into the above formulae, so that they may be of any convenient value, which is of great advantage in simplifying the manufacture of the potentiometer.' Contrast this condition with that of the temperature correcting circuit of Fig. 9 covering the range of only +0.1 to +9.5 pH.

summarizing The potentiometer-circuit of Fig. 1 has the following advantages:

(1) It delivers a potential that varies linearly with slider position;

(2) It reverses potential at any predetermined point on the slide resistor;

(3) It maintains the potential of any predetermined point on the potentiometer at a constant value regardless of the setting of temperature compensation switch |06 after restandardization;

(4) It shifts the point of zero potential and' changes the 'slope of the pH scale vs. E. M. F.

characteristic in response to adjustment for tem,

perature compensation whereby correction can be made for the temperature .vs. E. M. F. coefficient of a glass electrode-calomel electrode cell for all pH values between 1.0 and +9.0 in such manner that the pH scale on the potentiometer is direct reading for all temperatures;

(5) It employs only one calibrated and tapped resistor Ill3 and only one temperature compensating switch |06;

(6) It employs a slide resistor-89 having no tap connections thereon;

(7) It eliminates any necessity for avoiding end coil resistance in the slide resistor 89 beyond the right and left scale limits of the potentiometer,

With respect to advantage (7) illustrated above, it should be noted that in a system of the type disclosed in Fig. 9 there must be no resistance between the left end of the slide resistor (the point corresponding to a pH scale reading of +0.1) and the end of resistor 92 which is connected thereto.

Stated in a dierent Way, the potentiometer circuit of Fig.-1 provides temperature compensation without:

(1) Extra electrical or mechanical connections to the plain potentiometer winding or disc;

(2) A plurality of pH scales;

(3) Additional sliding contacts on the potentiometer slide resistor;

(4) Readjustments of index (usually a hair line) for reading the pH scale;

(5) Switches or other equipment actuated me-,. chanically bythe potentiometer disc or pH scale.

By avoiding the foregoing defects, the accuracy of the4 instrument is increased and the'expense of building and maintaining it reduced.v

Measuring values vof pH above +9.5 pH' The change in E. M. F. witnpnang'e in salue tion pH of a glass electrode-calomel electrode cell' is linear only up to a pH of +95. BetweenpH values of |9 5 and +11.1 the E. M. F.'rises at a successively reduced rate-per unit vcha-nge in a '7 linear although the slope of the pH in E. F.

characteristic above 11.1 pH is less than. below 9.1 pH. Hence as shown in Fig.` 5, the pH vs.

`E. M. F. characteristic ior any given temperaany of the potentiometer circuits discussed from 9.5 up to 12.5 for 'any given temperature, by gradually reducing the distances` between the graduations on the pH scale of the potentiometer over-the range 9.5 pH to 11.1 pH-(as shown-in Fig. 16) and making the spacing between graduations uniform over the pH range 11.1 to 1.2.5','but less than that over the range 1.0 to +95. How. ever, when temperature correction is laccom--v plished electrically as previously described, absolute accuracy is only obtained over the range 1.'0 pH to +95 pH. This willbe apparent from observation of the fact that, in eifect, the temperature correctionv circuits described rotate the E.' M. F. vs. scale deflection characteristic of the potentiometer about the point 0.4475 volt until the straight portion of the potentiometer characteristic between 1.0 pH and v+95 pH co- .incides exactly with the corresponding portion of the .pH vs.--E. M. F. characteristic of the 'cell tobe measured, whereas the portion of any one of the characteristics in Fig. 15 beyond pH +95 cannot be made to coincide exactly with the corresponding portion of another of the curves merely by rotation about the 0.4475 volt point.

Fortunately, however, I have discovered that the errors introducedin the range above'..+9.5 pH by employing the1 temperature compensating circuits described are so slight that they can be disregarded and it is practicable to.y design the,v

potentiometer to cover thepH range up'to at" least 12.5 pH units, which at presentl appears to be near the upper limit of accuracy of glass electrode cells.

As an examplefin a potentiometer in 'accordance with Fig. l, if the'pH scale is calibrated to give exact pH readings over the entire range from -`1.0 pH to +125 p H at a temperature of Circuits for reducind errors resulting'fro leakage currents Referring to Fig. l, it will be observed that when the switch .I3 is thrown to connect .the potentiometer 55 and test cell I in series across the input of the amplifier, the lead 9 extending fromA the glass electrode 3 is connected through switch' contacts .II and I5 and lead I9 to the control grid 2| of input tube 23. The'glass electrode 3 may have a resistance from about 40 megohms up to about 500 megohms. Thisis an enormous resistanceand itis obvious that very little leakage vof current fromth'e circuit elements 3, 9,

II, I5, I9 and 2I can betolerated if the full po" tential of the cell I is to'be impressed upon the grid 2l. Q

The aforementioned application Ser. No.

'148,048 describes methods of. reducing leakage from the grid 2l Within the tube itself and also describes methods of supporting the elements 9,

II, I5 and i5 from remaining portions oi the apparatus so as to obtain ahigh insulation gresis'tance.

those elements -are intercepted by a conducting body that is at the same potential -as the eleof conductor 12.

ments. Since. current can iiow between two points only in response to a difference in potential between those points, it follows that a balance no leakage can occur from the elements 3,A

9, II, I5 and I9.

The foregoing result is achieved by the simple expedient of grounding the lead 12 to the metal case Aand frame of the instrument instead of grounding the cathodes of the tubes as would be'done if usual amplifier practice were followed.

Leed 12 is eenneeted through switch s1 te the back contact I1 of switch I3 and to the junction ofresistors 92 and 93 which, when the po` tentiometer is adjustedto yield a potential equal andopposite to that'of the test cell I, is at the same potential as' contact II. This method oi reducing leakage current is clarified in Fig. 11 in which the various'insulators necessary'to mechanically support the elements3, 9; I I and I5, from the metal frame III are schematically illustrated. Thus in an acidimeter as actually constructed, insulating supports II3, II5, II1 and IIB, are employed to support the switch contact I5, switch contact II, the lead 9, and .the vglass electrode 3, respectively. The leakage path from the grid terminal of tube 23 to ground over the tube'envelope is indicated as a resistance I 2I.

' It will be apparent from Fig. 1l that in order for any foreign potential to be impressed upon any one of the elements I9, I5, II,'9 or 3, through the insulating supports of those elements, such potential would iirst have to be impressed upon the metal frame III and, since the lat-ter is' directly connected to the conductor l12, it'will be impossiblegior any foreign potential to produce a leakage .current through or over any of the insulating supports .II3, I I5, II1 or IIS, tending ,to change the potential of the elements supportedthereby or connected thereto from'the potential The improvement obtained by grounding. the circuit of Fig.`1 at the conductor 12 instead of at the iilamens of the tubes, as was done in the system ofthe Beckman and Frackerapplication, v

' serial No. 748,048 previously referred to, may b'e cussed later, omitted),' when the potentiometerl calculated as follows: If the system were grounded at the cathodes of the tubes, then the potenl tial diierence at balanceA between the" grounded frame of the apparatus and the elements i9, I5,

II', 9 and 3 would .be practically 3 volts with new A batteries and the average type 32 tube which is employed for-the 'tube 23. In the present circuit of Fig. 1 (with the resistor 51, which will be dis- 55 is adjusted to balance the-potential of the cell I within millivolt, which is easilydone,

then the potential difference between the grounded frame and the elements I9,V I5, II, 9 and 3' will be in the neighborhood of 1/2 millivolt.

Assuming that the insulation resistances are the same in eachinstance, the eiective insulacurred when balance.

tion has been improved by a factor equal to the ratio o! Y or 6000 times, a distinctly valuable improvement and one obtained simply by altering one wire connection.

'Ehe circuit of Fig. 1 also greatly reduces the diiliculty of maintaining a high leakage resistance ofthe insulators positioned adjacent the test cell I which is preferably enclosed in a compartment where high humidities are encountered. If the circuit were grounded at the cathodes o1' the tubes in accordance with the practice disclosed in the aforementioned prior application, the effective insulation resistance would be relatively independent of the potentiometer setting (since the biasing potential of approximately 3V volts existing between the cathodes and the conductor 12 is large as compared to the potential developed by potentiometer 55)- although the vWorst condition would occur when the potenf tiometer was set to either +0.25 volt when the glass electrode and calomel electrode were delivering their maximum E. M. F. of 0.5 volt. In the latter case the potential between the grounded frame of the apparatus and the con- A ductor I 9 would be approximately 3.7 volts.

occurs when the potential diil'erence between the Y potentiometer and the test cell reaches its maxlmumof 0.71 volt, in which case the eiective insulation resistance would be about 5.3 times as great as withthe previous grounding system and would still be approximately 4.2 times higher than the best insulation resistance obtainable with the previous circuit.

However, the currentleakage when the circuit of Fig. 1 is unbalanced is so slight that it does 1 not affect the operation when ordinary glass electrodes are employed. It an extremely high resistance micro-electrode were to be used, then it would be desirable to rst balance the'po entiometer against the test cell and then allo several minutes to elap'se before taking the final reading to permit time `for the disappearance of any pos.

sible polarization effects that might have octhe potentiometer was badly on.'

To prevent conilsion, in Fig. 11 the necessary physical supports for the potentiometer 55,/the

lead 85 'and' the'calomel electrode 5, have not been -indicated. 'I'he insulation requirements of the supports f or these elements are not severe,`

despite the fact that, at balance, Athe potentials of conductor 85 and electrode 5 diier from ground potential by the E. M. F..produced by the potentiometer, and the test solution in which electrodes z end s are immersed is' et e potential differing I from that of electrode 5. The reason for this is that the potentiometer 55 has a relatively low resistance o1' 'theordei of magnitude vo1' .1000

ohms,and hence the conductor. and calomel Vcell 5 are at .a very low impedance level. with respect to ground, as compared to the impedance level of .the glass electrode and the elements connccted thereto. I1' the maximum-allowable'error in the potentiometer circuit isvheld to approxibe easily obtained. q

Circuit lfor reducing swinging of the meter needle when the comparing switch is actuated In a' conventional D. C. amplifier of a type containing no inductances or capacities, as illustrated in the aforementioned previous application; the voutput current of the amplifier changes rapidly in response to a change in the grid potential of the input tube. The result is that whenever the switch inthe former system corresponding to switch I3 in Fig. 1 of the present system is actuated to shift the transfer contact away from the back contact and against the front contact, there is a moment during the transfer of the movable contact when it is connected to neither the back nor front' contact, thereby leaving the grid of the iirst tube floating, which allows the plate current of the first tube to rise and the plate current of the output tube to fall,

permitting the needle ofthe milliammeter to fall v to zero. As a result, .every time the comparing switch in the system of the prior application is actuated the milliammeter needle shows a deflection that is not only annoying to the operator but that may give rise to ampller drift unless the switch contacts are carefully adjusted so that the length of time that the grid circuit is open is short compared to the period of the output meter. The drift is caused by the fact that changes in the plate current of a type 30 tube, which is employed as the output tube, are relected as filament current changes and the readings of the output meter are very sensitive to alterations in the lament current of the input tube (a type 32 tube) because 'of the factthat the filament'is normally operated at a very low temperature. As a result,v a momentary 100% change in the plate' current of the output tube lmay cause very apreciable drift which is particularly annoying in that it begins` when readings are being taken.

It is possible to reduce the drift by adjusting the contacts ofthe transfer switch very carefully so that the moving contact is clear of thefixed contacts for only a very short interval of time, but such delicate adjustments are undesirable.

This condition might be prevented by so arranging the transfer switch as to cause the transfer contact to make on the front contact before it broke from the back contact. Such an ar. rangement is out of the question, however, as it would momentarily shortthe test cell and'po-` larize it.

With conhectional D. C. amplifier construction,

undesired meter deflection and amplifier drift would also `occur when switch 61 (Fig. 1) is actu. ated,*during the standardizing operation. vIt has been suggested that the polarizationl and drift eiiects might be avoided during the standardizing operation, by employing a switch l'that made contact before the break, and connecting a 50,000

ohm resistance effectively in series with the standard cell 99. However, such an arrangementl has been found unsatisfactory because of errorsy introduced by variable feed back from the B battery leads through the wiring insulation, the error amounting at times to 6 millivolts in the v denser were shunted across potentiometer standardization. As an example, if 1A, millivolt is set as the maximum error permissible then the current necessary to produce this E. M. F. through a 50,000 ohm resistor is which equals2500 megohms, an `ins ulation requirement that is.diiiicu1t toV meet..

In accordance with' the present invention, both drift and excessive meter fluctuations are elimjustment of the contacts of switch I3, by increas- 1 ing the time constant of the amplifier. This is effected by connecting a condenser |3I betweenv the grid and filament of the output tube 4I. The condenser `|3I acts as a reservoir to absorb or release current from or to the conductor |33 and the grid 39 of tubell. thereby P'lleventing sudden changes in the potential of the grid.

Thus referring to Fig. 12, the square bottom curve |34 represents the anode current of tube 4I plotted against time as grid 2| of tube 23 is disconnected from the grid return lead l2 for the interval of time i, with the condenser |3I disconnected. It will be noted that the current drops immediately to a low value and thereafter rises sharply to the original value, when the grid of tube 23 is again connected to the grid return lead l2. Such achange in the output current causes a corresponding fluctuation of the meter Il in the output circuit of tube 4I.

The use of the condenserv |3I tends to prevent sudden variation in the grid potential of tube Iii, and causes its anode current to fall and rise more gradually. As a result, when the; grid circuit of tube 23 is momentarily opened for the 4 ,system when the standardizing switch 8l is actuated. Another objection is that it would' be very dimcuit to produce a 'condenser having insulainated, without the necessity of a' delicate adtion and.leakage resistance high as compared to that of a glass electrode cell. In addition, the charging current drawn by the condenser at the instant the glass electrode-cell is connected thereto might be 'sufficient to polarize the cell. Dis*- placement currents within the condenser might also affect the operation of the system if the conresistance input circuitof the amplier;

The capacity of condenser |3| that must be employed depends, of course, upon the values of the other elements in thevcircuit, particularly` Where the resistor 33 4.

that of the. resistor 33. has a value of about 20 megohms, which is in acycordance with present practice, .a condenser of .005 to '.01' microfarad gives a time constant of the order of magnitude of 1-,th second, which is the extremely highY A ample to 'take care of considerable variation in the contact adjustments of switch I3. A mica condenser should be used as 'such condensers have an exceedingly high insulation and leakage resistance, are permanent in capacity and show no dielectric absorption.

Obviously, many variations and minor renea plete electrical low pass lter circuit containing ,anode circuit of tube 23.

'pling resistor 33.

inductance as well as capacity. However, such further refinement is not justified in the acidimeter circuit disclosed.

Adjustment ,for

varying sensitivity of -the amplifier The usual methods of controlling amplifier gain ork sensitivity employed in A. C. amplifiers are not as a rule adaptable to D. C. amplifiers unless complicated multiple adjustments are permissible, since any change in the operating parameters of a D. C. amplifier will generally affect the steady reading of the indicating instrument used in the output circuit. Y

In the original acidimeter disclosed in th' copending application Serial No. 748,048, no provision was made for varying the sensitivity of the amplifier. The amplifier therein employed was sensitive to achange in grid voltage of the iirst tube of about 0.1 millivolt and was arranged to give a sensitivity of not less than-1.5 millimeters movement of the needle on the indicating milliammeter fora change in grid voltage on the first tube of -one millivolt.

In the 'presentacidimeter a sensitivityrof 1grd the former value, or 0.5 milliammeter needle `movement per millivolt change in grid potential of the input tube has been deemed suilicient. It has also been found that sensitivity control 4is desirable on account of recently 'encountered variations in sensitivity of the type 32 tubes employed in the first stage. Therefore, to render all acdimeters uniformly sensitivesome form of gain control is necessary.

Oifhand, it would appear that there are many Possible and vobvious methods of reducing the gain of the amplifier circuit shown in Fig. 1. One such method would be to reduce thevalue of the 20 megohm coupling resistor 3,3 in the This would have .the advantage of not only reducing the gain but increasing the stability of the circuit and reducing the magnitude of interference produced by external A. C. nelds. Increase of stability would result from the fact that parallel leakage paths throughthe C battery and insulation would have les effect on the indicating meter. Stability would also be improved by virtue of the fact that since the 15 volt C biasbattery 31 has a Even more com- I `eiect of external A. C. fields on the circuit as then it would notbe necessary to so completely shield the apparatus, which would reduce the cost of manufacturing.

However, offsetting these advantages resulting from a reduction in the value of resistor 33 is the serious disadvantage that it would necessitate an increase in thevalue of the biasing battery 31 which would increasethe size of the case necessary to hold the apparatus. It,is desirable to make the apparatus portable and mount itin as compact a case as is compatible withvreliable and accurate operation.

Another rather obvious method Jo1' reducing the i the B battery in the circuit disclosed.

Still another'method of reducing the gain is to reduce the filament potential on the tubes but this is objectionable because it Vmore than proportionately reduces the useful life of the A battery and would *necessitate the introduction of a variable resistance with attendant contact difliculties into the filament circuit, which is extremely sensitive to any variation in current.

' 'Gain reduction by the introduction of resistance in'series with the grid of the output tube 4| is possible but is objectionable because it would necessitate an excessively high value of resistance and a readjustment of the grid biasing potential.

Gain reduction could also be eiected by introducing resistance into the plate circuit of tube 4l in series with the meter. 41 'but to achieve an appreciable reduction in gain the resistance wouldv vhave to be so high that an indicating meter of smaller current-range more delicate and costly than the one employed (having a range of 0-1 milliampere) would be required. y

Still another method of reducing gain .that has been employed with some success is to divide the 20Vmegohm resistor 33 into two 10 megohm resistors connected in. series and connecting the' lead 133 from the grid 38 to the junction of the two resistors instead of to the upper end of the resistor v33, as shown in Fig. 1. This method, however, has the objection that, in order to maintain the mean Vplate current of the tube 4I. at the desired value, it requires either-that a' larger AC battery 31 be employed, or that the negative lead of the B battery 35 be disconnected from the positive terminal of the A battery 49 and connected between the two cells of the A battery, thereby reducing the effective potential applied to the plate of theV output tube 4| (an undesirable change).

A method of gain reduction that is found to be more satisfactory than any of the foregoing' methods consists in connecting a variable resistor |31 in seriesvwith the screen grid 21 of the input tube 23. The resistor 131 should preferably have a rather large resistance but it need have little current carrying capacity. A resistor havingavalueofaboutS to 6megohmsreduces the sensitivity to pproximately the desired 'loY quires a charging current to ow through the cou- 3881116.

If the value of this resistor 33 were reduced the voltage across it caused by the charging current would be proportionately reduced. It would be advantageous to reduce the This method of reducing or controlling gain is generally' considered objectionable in conventional A. C. ampliers. Thus, although it is comman to adjust the gain in A. C. ampliiiers emas' a low resistance potentiometer) which does not introduce a large resistance in series with the screen grid. This necessity of maintaining ,fia low resistance in the screen grid circuit of' a conventional A. C. amplier is due, in part at least, -to the fact. thatin such amplifiers the anode is always operated at a higher potential\ than the screen grid, under which conditions the current drawnby the screen grid is found to vary widely in dilierent tubes of the same type, ,as purchased on the open market. In my system, on the contrary, the screen grid operates at a higher potential than the anode, and the gain control effect is due far more to the presence of 4the resistance, which changes the static screen grid characteristic to a dynamic one, than to the reduction in thevoltage applied to the screen grid. As a matter of fact, it is objectionable vin the amplier circuit of Fig. 1 to reduce the potential applied to the screen grid to an extent suicient to appreciably reduce the gain, because the region o'f maximum mutual con'- ductance of the type 32 tube is determined by the ratio of the potentials` applied to the control grid and the screen grid, respectively, and the negative bias on the control grid must be in excess of a certain minimum value in order to insure that the grid always will be sufficiently negative to maintain the input resistance-of the tube' high despite positive potentials applied to the grid as a result of the potentiometer 55 being out of balance with respect to the test cell potential.

The use'of the resistor |31 so flanens out the overall dynamic characteristic of the entire amthe grid current of the amplifier will cause errors '.will be explained with. reference to Fig. 1li,` in

plfler that the region of uniform sensitivity is considerably inereased'on the output meter. It

' also has the desirable eiect of maintaining a much more uniform amplifier sensitivity throughout the life of the batteries.

Method of eliminating errors resulting ,from grid current in the input tube of the amplifier As has been explained in detail inthe previous I application Serial No. 748,043, an important advantage of using an amplifier in combination with a milliammeter (instead of using a galvanometer alone) Ior'pH measurements with a glas electrode cell, is that the amplifier has an ex remely high input resistance. However, even with the best design the input resistance of a vacuum tube amplier, particularly when using ordinary comwith extremely high resistance glass electrodes if compensation therefor is not provided.-

The manner in which the-errors are produced which the equivalent circuit of a test cell having a glass electrode is represented by a generator |4I developing apotential Ec, in series'with a resistance |43, which may be assumed to-be 190 megohms. The potentiometer connected in series Awith' thetest cell is represented as a generator |45 f developing a potential Ep and having negligible resistancagA battery |41 represents the source of biasing potential on the grid 2| of the ampliiier input tube fand the -resistor |49 represents the input or-grid-cathode resistance R of the tube. Insulation leakage resistances have been disregarded in Fig. 14.

As shown in the circuit of Fig. 14, E1 equals amperes, which is more'nearly of the order of magnitude of the gridy currents encountered among certain type 32 tubes'.

If I is to be kept to about 10" amperes, the value of R must be of the order of 2 1011 ohms. However, microA glass electrodes have been developed which have resistances of about 500 megohms and, using such electrodes, if the maximum error isnot to exceed .05 pH the current I must be kept to amperes. Since this value of current is smaller than the input current of the amplifier disclosed l in Fig. 1 for certain type 32 tubes, some means for increasing the eifective input resistance of the tube 23 is desirable, particularly since this could be one of the largest sources of error in pH determination using the present acldimeter with high resistance electrodes.

. I have discovered that the error resulting from grid current may be effectively compensated for by using the resistor 51 (Fig. l) in series with the contact I1 of switch I3, the resistor 51 having a resistance approximately equal to that of the glass electrode 3. The manner in which resistor 51 functions to reduce the error caused by grid current will now be explained.

Grid current flows continually and is unaffected in magnitude by the presence of ordinary values of high resistance in series with the grid.v Furthermore, at balance the potential `driving the grid current remains thesame regardless of -whether the switch I3 is in the normal oroperated position since in either case the return circuit from the lgrid is to the grid return lead 12, which ls biased with a constant negative potential relative to the cathode of tube 23. It follows then that if the resistor 51 has exactly the same value of resistance as the glass electrode 3, then the potential drop occasioned by grid current between the grid return lead 12 and the contact I5 of L switch I3 is the same regardless of whether the -contact I5 is resting against contact I1 or contact II.

It is not necessary that the value'of resistor 51 exactly equal the glass electrode resistance for in the case of a 100 megohm glass electrode with the maximumv error set at l/ millivolt and a grid current of 2.5X10-11 amperes (which would give in Fig. 1, if resistor 51 were eliminated, a grid current error of 21/2 millivolts) a value of resistance for resistor 51, within s -within the specified limit. For lower resistance ance limits to keep the maximum error within a speciiied value for a size of resistor 51`which may be standardized in the acidimeter construction. Areduction in the measuring error due to grid current by a factor of at least 5 is very readily gained by the use of the resistor 51 and much greater improvement can, of course, be had by using lower resistance glass electrodes and/or allowing, for instance, a lower percentage variation between the resistances of the glass electrode and of the resistor 51.

The described method of decreasing the effect of grid current possesses further advantages from a manufacturing viewpoint in that it allows much wider tolerances in grid current among dierent type 32 tubes and imposes less severe requirements on the filament current adjusting resistor 53 .connected across the lament of the tube 23. In fact, it becomes possible, by using the balancing resistor 51, to allow the grid current limit to be established, not by the extremely small voltage drop through the glass electrode resistance as was formerly necessary, but by the much larger current necessary to polarize the cell which easily may be more than 1000 times greater. This means that greater latitude in selection oi circuits is possible, thereby increasing amplifier stability and reducing manufacturing costs through elimination of inspection operations.

The introduction of the resistor 51 reduces the eiectiveness of the ground on the lead 12 in preventing leakage current from the circuit elements I9, l5, Il and 3, the loss in effectiveness being dependent on the potential drop between conductor 12 and the contactl I1 of Fig. 1 occasioned bythe resistor 51. In the previous discussion of the reduction in leakage current obtained by grounding conductor 12 instead of the cathode of. the tubes," it was assumed that the resistor 51 was not employed. However, even if a voltage of, for instance, 21A; millivolts, which may be a value realized in practice, exists across resistor 51, then instead of the'factor of improvement resulting from the grounding of the lead 12 being 6000 as before, it will be dropped-to ooox of" 1200,

lbut the overall accuracy is still greatly improved,

shown will occur to those skilled in the art and the invention is not limited to the particular circuits disclosed, but only to the extent set forth in the appended claims. x

I claim:

1. In a. calibrated potentiometer-for balancing the E. M. F. oi and indicating directly at a plurality of temperatures the pH of a. test cell,.-

which cell has a constant potential other 'than zero at all said temperatures at one value of pH but which has a potential which varies linearly with temperature change at other values of pH, a slide resistor calibrated in pH units, means for energizing said slide resistor, and calibrated means for electrically varying the scale vs.

E. M. F. characteristic of said slide resistorV to correspond to the pH vs. E. M. F. characteristics of the cell at diilerent temperatures.

2. A circuit as dened in claim 1, in which said means for supplying said predetermined constant potential comprises a battery in series with a variable resistor, and means for indicating when the total potential across the battery and variable resistor is adjusted lto said predetermined value.

3. A potentiometer for indicating directly at any one of a plurality of temperatures the pH equivalent o f the potential of a pH test cell4 which cell has a constant potential other than zero for all said temperatures at one value of pH but which has a potentialwhich varies linearly with temperature change at other values of pH, said potentiometer comprising a slide resistor graduated in pH units and a slide therefor, a first E. M. F. output terminal connected to said slide, means for supplying a predetermined potential to the ends of said slide resistor, a second E. M. F. output terminal, and means connected to said slide resistor for deriving therefrom and applying to said second output terminal a potential xed with respect to the potential on the point of said slide resistor corresponding to said one value of pH, means for varying the potential applied to the ends of said slide resistor by predetermined amounts to vary the slope ofk the E. M. F. vs. pH scale characteristic of the slide resistor and cause it to correspond to the slope equivalent of the potential of a pH test cell, which` cell has a constant potential other than zero for all said temperatures at one value of pH but which has a potential which varies linearly with temperature change at other values of pH said potentiometer comprising a slide `resistor graduated in pH units and af slide therefor, a rst'E. M. F. output terminal connected to said slide, a fiirst variable `resistor and a source of constantE. M. F. connected in series with said slideresistor, a second resistor in shunt to said slide resistor and -a second E. M. F. output terminal connectedV` to a tap on said second resistor 'so positioned thereon as to provide a potential fixed with respect to` that of the point on said slide resistor corresponding. to said one value of pH, means for varying said first variable resistor to vary the current in said slide resistor and thereby vary the slope of the E. M. F. vs.,pH scale characteristic-of the slide resistor tocor- `respond lto the' different slopes of the F.

vs. pH characteristics of the cell at different temperatures, and means for varying the ratio of the resistances of the portions of said second resisterj onv opposite .sides of the tap thereon to correct for shifts in the potential `o'f the point on 'said slidqe resistor corresponding to Ksaid one value of .pH produced bysaid variations in the current in said slide resistor. y

5. A potentiometer-for indicating directly at any one of a plurality of temperatures the pH equivalent of` the potentialof a pH test cell, which cell has a constant potential other than zero for all said temperatures at one value of pH but which has a potential whichl varies linearly with temperature change at other values of pH, said potentiometer comprising a slideresistor graduated in pHunits and a slide therefor, a first E. M. F. output terminal connected to said slide, a source of current' connected to the ends of said slide resistor for producing a potential drop therein, a second output terminal, and means for deriving from said source a constant reference potential and applying it to said second output terminal, means for maintaining the point on said slide resistor corresponding to said one value of pH at a-xed potential with respect to said constant reference Vpotential despite variations in current in said slide resistor, and means for varying the current through the said slide resistor by predetermined amounts to vary the slope of the E. M. F. vs. pH scale characteristic of the slide resistor to correspond to the different slopes-of the E. M. F. vs. pH characteristics of the cell at different temperatures.

6. A potentiometer for indicating directly at any one of a plurality of temperatures the pH equivalent of the potential of a pH' test cell,

which cell has a constant potential other than zero for all said temperatures at one value of pH but which has a potential which varies linearly withtemperature change at other values of pH, said potentiometer comprising a slide resistor graduated in pH units and a slide therefor, a iirst E.' M. F. output terminal connected to said slide, a temperature calibrated variable resistor having one end connected to one end of said slide resistor, a source of current of constant potential connected between the other end of said calibrated resistor and the other end of said slide resistor, a tapped resistor connected across said source and a second E. M. F. output terminal connected to the tap thereon, said tap being so positioned as to have a potential differing from the potential of the said other end of the slide resistor by the potential of said cell at said one value of` pH, and said temperature calibrated resistor being so proportioned with respect to the slide resistor as to vary the current therethrough by such amounts as to vary the slope of the E. M. F. vs.'pHl scale characteristic of the slide resistor to correspond to the different slopes of the F. vs. pI-I characteristic of the cell at diierent temperatures.

7. A potentiometer for indicating directly at any one of a plurailty of temperatures the pH equivalent of the potential of a pH test cell,

, point on said slide resistor corresponding lto said.

which cell has a constant potential other than zero for all said temperatures at one value of pH but which has a potential which varies linearly with temperature change at other values of pH, said potentiometer comprising a slide resistor graduated in pH units and a slide therefor, a rst F. output terminal connected to said slide, a temperature calibrated resistor having one end connected to one end of said slide resistor, a source of current of constant potential p connected between the other end of said calibrated resistor and the other end'of said slide resistor, a tapped resistor of large value relative to said slide resistor connected between said other end of said temperature calibrated resistor and a one value of pH, a second E. M. F. output -terminal connected to the tap on said tapped resistor, said tap being so positioned as to have a potential diiering from the potential of said point ori said slide resistor by the potential of said cell at said one value vof pH, and said temperature calibrated resistor being so proportioned teristic of the slide resistor to correspond to the different slopes of the E. M. F. vs. pH characteristics of the cell at predetermined diierent temperatures.

8. A potentiometerfor indicating directly at vany one of a plurality of temperatures the pH equivalent of the potential of a pH test cell,

which cell has a constant potentialv otherA than zero for all said temperatures at one value of arly with temperature change at other values of pH, said potentiometer comprising a slide' resistor graduated in pH units and a slide therefor, a rst E. M. F. output terminal connected to said slide, a temperature calibrated variable resistor having one end connected to one end vof said slide resistor, a source of current of predetermined potential connected "between the other end of said calibrated resistor and the other end of said slide resistor, a first tapped resistor of total resistance of the saineI order of magnitude as the slide resistor connected in shunt to said slide resistor, a second tapped resistor of large i value relative to said rst tapped resistor confis" ' pH but' which has a potential which varies linesecond resistor, the .latter being so located as to'have a potential differing from the potential of the point on the slide resistor corresponding to said one value of pH by the potential of said cell at said one value of pH, and said temperature calibrated lresistor being sol proportioned with respect to the slide resistor'as'to vary thecurrent therethrough by such amounts .as to vary the slope of the E. M. F. vs. pH scale characteristic of the slide resistor to correspond to the diierent slopes of the E. M. F. vs. pH char` cathode, indicating meansv connected to the out'-y put of said amplier responsive to potential applied to said input terminals, and means comprising contacting elements for connecting a high resistance circuit the potential of which `is to be measured in series opposition with a pctentiometer across.the input terminals of said amplifier, withthe circuit to be measured positioned adjacent the grid terminal of the ampliner, in which the apparatus comprises ametallic supporting frame electrically connected to said other input terminal and in which the. conducting elements connecting the grid of said tube to v the circuit to be measured are supported by insul'ating elements extending directly from said .metallic frame.

10. An/acidimeter for measuring the pH of sojlutions', comprising an amplifier containing a vacuum tube having a grid and cathode, a pair of input` terminals, means connecting one of said input terminals to said grid, and means including a source of grid biasing potential connecting ing means connected 'to the output of said amthe other input'terminal to saidcathode, indicatadapted to extend into the solution therein, one

of 'said electrodes having an extremely high resistance, a potentiometer having a resistance low as compared to that of said one electrode, means for connecting said electrodes in series opposition with,l said potentiometer across the input Aterminals of said ampliiler, with said one electrode adjacent the grid terminal of the amplifier, in which the apparatus comprises a conductive supporting frame electrically connected to said other input terminal and in whichthe conducting-elements,` connecting the grid of said tube to said one 'electrode of said ,cell and said one electrode are supported by insulating elements extending from said conductive frame whereby said frame intercepts all insulation paths between said conducting elements and other elements in said apparatus at potentials different from that of said frame.

11. In D. C. testing apparatus, a D. C. vacuum tube ampliiler having an indicating meter in its output circuit and switch means for successively connecting to the input circuit of the amplifier different circuits, the potentials of which are to be compared with each other, in combination with means for increasing the time constant of the amplifier whereby sudden` changes in the potential applied to the input circuit do not produce violent changes in said indicating meter.

12. In combination, a D. C. amplier comprising a iirst tube and a second tube each having a cathode, grid and anode, switch means for switching the grid of the first tube from connection direct to its cathode lto connection thereto through a sourcegof potential to be compared, said switch disconnecting said grid completely from the cathode during transition from one connection to theother, a source oi.' anode potential anda coupling resistor connected between the cathode and-anode of said rst tube, a source 'of biasing connection connecting 'the anode of the nrst tube to the grid of the sec'ond tube, a source j y of potential. connected between the anode and cathode of said second tube,A means for indicating variations in the anode current of said secmeasured, a resistor of the'same approximate resistance as that of the circuit to be measured, switching means for alternately connecting said potentiometer and the circuit to be tested in series with each other across said input terminals andconnecting said input terminals together' through said resistor, said voltage indicating means including a source of potential in series with said input terminals and having a substantially constant high input resistance.

14. Apparatus for measuring potentials inhigh resistance circuits, comprising in combination a D. C. amplier having current indicating means connected in its output circuit, and comprising an'input tube having a cathode and agrld with means for impressing a predetermined negative biasing potential in series between said cathode and grid, a potentiometer adjustable to supply a potential equal to that of circuit to be measured, a resistor of the same approximate resista/nce as that of the circuit to be measured, switch means for alternately connecting said potentiometer and the circuit to be'tested in series with'each other and with said source of biasing potential between said grid and cathode or connecting said resistor 'and source of biasing potential alone between said grid and cathode, whereby regardless of the position of said switching means the resistance in series with said source of biasing potential between said grid and cathode is substantially the same.

f HENRY-"E, FRACKER. 

